Maintained by Robin Tecon, microbiologist and postdoctoral researcher at the Swiss Federal Institute of Technology Zürich. This blog is about bacteria (and other microbes) and the scientists who study them.

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Wednesday, November 07, 2012

Plant pathogen focus: black Sigatoka as a worldwide threat to banana

Since I work in the department of plant pathology at UCD—and
even though I am not a plant pathologist myself!—I decided to start a series of
posts on microbes that cause plant disease, focusing on stories that are of
economic and societal importance.

A couple of weeks ago, one of my colleague—him
a true plant pathologist—, managed to scare me by claiming during a talk that banana
could disappear in the not-so-distant future! The culprit? The fungus Mycosphaerella fijiensis: This ascomycete
causes a disease (black Sigatoka) that damages the leaves of banana trees and
reduces photosynthesis. Moreover, the fungus triggers a premature ripening that
spoils the fruit. Together these effects of black Sigatoka provoke the loss of
50% or more of the fruit production.

[I learnt a lot about black Sigatoka in an
online article by Randy Ploetz on the website of the American Phytopathological
Society (APS). When no other source is explicit, the information in this post
comes from the Ploetz article. In general, the APS website is a great starting
point for everything related to plant pathology!]

Now, the prospect of having to say farewell to bananas was
sad enough for me, but then I learned that banana was much more than an
enjoyable fruit after lunch: Not only is it the most important fruit in terms
of production, but it is also an essential food crop, coming right after rice,
wheat and maize (Churchill, 2011). It is worth knowing that only about 10% of
the world production is exported; the vast majority of bananas (including
plantains) are consumed in the countries that produce them, and it is thus an
essential part of the diet of many poor inhabitants. Originally from Southeast
Asia, bananas are now cultivated worldwide, including in India, Uganda,
Ecuador, Brazil and Colombia (Marín, 2005). What is also interesting to
know is that there is no season of banana harvest, since banana trees produce
all year long!

Banana trees are susceptible to several diseases, most of
them caused by fungi. Among these diseases, the current most threatening one is
the aforementioned black Sigatoka, also known as black leaf streak. Mycosphaerella fijiensis, the causative
agent of the disease, was first observed in the Sigatoka Valley of Fiji (hence
its name) in the sixties. Since then, black Sigatoka has spread to America, the
Caribbean and Africa… and a considerable amount of money has been spent for
years in its control by fungicides. And, unsurprisingly, the appearance of resistances followed. Today, to overcome resistance, growers
apply mixtures of fungicides that have different modes of action. These
treatments are costly, and it is estimated that they are responsible for up to
20% of the price we pay the banana on the shelf.

Efforts have led to the development of new plant hybrids
that are resistant to black Sigatoka, but at present these new banana cultivars
(cultivated varieties) do not produce fruits that meet exportation standards. Scientific
research on the disease is very active, but it is complicated by the fact that Mycosphaerella fijiensis does not grow
well in the laboratory. Despite the fungus’ reluctance to lab cultivation, much
information has been obtained on M.
fijiensis’ biochemistry, and its complete genome sequence is available since
2010. It has to be noted, in addition, that the banana genome has been completed this year. These developments will help research, undoubtedly, but the
solutions to the threat caused by black Sigatoka are neither simple nor
immediate. As far as I know there is no attempt to produce genetically-modified
banana that would be resistant to black Sigatoka (on the other hand, GM bananas are currently developed in Uganda to resist to another banana disease caused by
the bacterium Xanthomonas campestris).
If the fungus resistance to chemical treatment increases, however, it might boost
the use of the new hybrids in the future, despite the lower quality of their
fruits.

The future is challenging for the culture of banana all over
the world, but hopefully growers and researchers will meet the challenge!